Multiple-wavelength opto-electronic device including a superlattice

ABSTRACT

A multiple-wavelength opto-electronic device may include a substrate and a plurality of active optical devices carried by the substrate and operating at different respective wavelengths. Each optical device may include a superlattice comprising a plurality of stacked groups of layers, and each group of layers may include a plurality of stacked semiconductor monolayers defining a base semiconductor portion and at least one non-semiconductor monolayer thereon.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor devices,and, more particularly, to opto-electronic devices and related methods.

BACKGROUND OF THE INVENTION

Structures and techniques have been proposed to enhance the performanceof semiconductor devices, such as by enhancing the mobility of thecharge carriers. For example, U.S. Patent Application No. 2003/0057416to Currie et al. discloses strained material layers of silicon,silicon-germanium, and relaxed silicon and also including impurity-freezones that would otherwise cause performance degradation. The resultingbiaxial strain in the upper silicon layer alters the carrier mobilitiesenabling higher speed and/or lower power devices. Published U.S. PatentApplication No. 2003/0034529 to Fitzgerald et al. discloses a CMOSinverter also based upon similar strained silicon technology.

U.S. Pat. No. 6,472,685 B2 to Takagi discloses a semiconductor deviceincluding a silicon and carbon layer sandwiched between silicon layersso that the conduction band and valence band of the second silicon layerreceive a tensile strain. Electrons having a smaller effective mass, andwhich have been induced by an electric field applied to the gateelectrode, are confined in the second silicon layer, thus, an n-channelMOSFET is asserted to have a higher mobility.

U.S. Pat. No. 4,937,204 to Ishibashi et al. discloses a superlattice inwhich a plurality of layers, less than eight monolayers, and containinga fraction or a binary compound semiconductor layers, are alternatelyand epitaxially grown. The direction of main current flow isperpendicular to the layers of the superlattice.

U.S. Pat. No. 5,357,119 to Wang et al. discloses a Si—Ge short periodsuperlattice with higher mobility achieved by reducing alloy scatteringin the superlattice. Along these lines, U.S. Pat. No. 5,683,934 toCandelaria discloses an enhanced mobility MOSFET including a channellayer comprising an alloy of silicon and a second materialsubstitutionally present in the silicon lattice at a percentage thatplaces the channel layer under tensile stress.

U.S. Pat. No. 5,216,262 to Tsu discloses a quantum well structurecomprising two barrier regions and a thin epitaxially grownsemiconductor layer sandwiched between the barriers. Each barrier regionconsists of alternate layers of SiO₂/Si with a thickness generally in arange of two to six monolayers. A much thicker section of silicon issandwiched between the barriers.

An article entitled “Phenomena in silicon nanostructure devices” also toTsu and published online Sep. 6, 2000 by Applied Physics and MaterialsScience & Processing, pp. 391-402 discloses a semiconductor-atomicsuperlattice (SAS) of silicon and oxygen. The Si/O superlattice isdisclosed as useful in silicon quantum and light-emitting devices. Inparticular, a green electroluminescence diode structure was constructedand tested. Current flow in the diode structure is vertical, that is,perpendicular to the layers of the SAS. The disclosed SAS may includesemiconductor layers separated by adsorbed species such as oxygen atoms,and CO molecules. The silicon growth beyond the adsorbed monolayer ofoxygen is described as epitaxial with a fairly low defect density. OneSAS structure included a 1.1 nm thick silicon portion that is abouteight atomic layers of silicon, and another structure had twice thisthickness of silicon. An article to Luo et al. entitled “Chemical Designof Direct-Gap Light-Emitting Silicon” published in Physical ReviewLetters, Vol. 89, No. 7 (Aug. 12, 2002) further discusses the lightemitting SAS structures of Tsu.

Published International Application WO 02/103,767 A1 to Wang, Tsu andLofgren, discloses a barrier building block of thin silicon and oxygen,carbon, nitrogen, phosphorous, antimony, arsenic or hydrogen to therebyreduce current flowing vertically through the lattice by more than fourorders of magnitude. The insulating layer/barrier layer allows for lowdefect epitaxial silicon to be deposited next to the insulating layer.

Published Great Britain Patent Application 2,347,520 to Mears et al.discloses that principles of Aperiodic Photonic Band-Gap (APBG)structures may be adapted for electronic bandgap engineering. Inparticular, the application discloses that material parameters, forexample, the location of band minima, effective mass, etc., can betailored to yield new aperiodic materials with desirable band-structurecharacteristics. Other parameters, such as electrical conductivity,thermal conductivity and dielectric permittivity or magneticpermeability are disclosed as also possible to be designed into thematerial.

Despite considerable efforts at materials engineering to increase themobility of charge carriers in semiconductor devices, there is still aneed for greater improvements. Greater mobility may increase devicespeed and/or reduce device power consumption. With greater mobility,device performance can also be maintained despite the continued shift tosmaller devices and new device configurations.

One particular application in which improved materials providing greatermobility may be desirable is optical devices. For example, typicaloptical detectors used in solar (i.e., photovoltaic) cells are made ofamorphous silicon and are thus relatively inefficient. As such, toprovide desired power output in many applications a relatively largesurface area has to be covered with such solar cells, which may not bepractical. Accordingly, it would be desirable to incorporate materialswith enhanced mobility in solar cells to improve efficiency thereof withreduced weight and/or surface area requirements.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to provide a multiple-wavelength opto-electronicdevice having enhanced mobility characteristics.

This and other objects, features, and advantages are provided by amultiple-wavelength opto-electronic device which may include a substrateand a plurality of active optical devices carried by the substrate andoperating at different respective wavelengths. Moreover, each opticaldevice may include a superlattice comprising a plurality of stackedgroups of layers, and each group of layers may include a plurality ofstacked semiconductor monolayers defining a base semiconductor portionand at least one non-semiconductor monolayer thereon. More particularly,the at least one non-semiconductor monolayer may be constrained within acrystal lattice of adjacent base semiconductor portions, and at leastsome semiconductor atoms from opposing base semiconductor portions maybe chemically bound together through the at least one non-semiconductormonolayer therebetween.

In addition, each active optical device may further include first andsecond semiconductor regions on opposing sides of the superlattice andhaving opposite conductivity types. The optical devices may be stackedin a vertical direction, as well as positioned laterally adjacent oneanother, for example. The plurality of superlattices may have differentnumbers of semiconductor monolayers in their respective basesemiconductor portions, for example.

By way of example, the active optical devices may be optical detectors.Moreover, the optical detectors may be configured to provide an outputequal to a sum of photocurrents therefrom, thereby providing anefficient solar cell arrangement. In other embodiments, the activeoptical devices may comprise optical transmitters, or both opticaldetectors and transmitters may be used in the same device.

Each superlattice may have a substantially direct bandgap. Moreover,each superlattice may also have a different respective bandgap. Themultiple-wavelength opto-electronic device may further include at leastone contact coupled to the plurality of active optical devices. Thesubstrate may comprise various materials including semiconductors andnon-semiconductors. In accordance with one exemplary embodiment, thesubstrate may comprise glass.

Additionally, each base semiconductor portion may comprise a basesemiconductor selected from the group consisting of Group IVsemiconductors, Group III-V semiconductors, and Group II-VIsemiconductors. By way of example, the base semiconductor portions maycomprise silicon. Also, each non-semiconductor monolayer may comprise anon-semiconductor selected from the group consisting of oxygen,nitrogen, fluorine, and carbon-oxygen, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a greatly enlarged schematic cross-sectional view of asuperlattice for use in a semiconductor device in accordance with thepresent invention.

FIG. 2 is a perspective schematic atomic diagram of a portion of thesuperlattice shown in FIG. 1.

FIG. 3 is a greatly enlarged schematic cross-sectional view of anotherembodiment of a superlattice in accordance with the invention.

FIG. 4A is a graph of the calculated band structure from the gamma point(G) for both bulk silicon as in the prior art, and for the 4/1 Si/Osuperlattice as shown in FIGS. 1-2.

FIG. 4B is a graph of the calculated band structure from the Z point forboth bulk silicon as in the prior art, and for the 4/1 Si/O superlatticeas shown in FIGS. 1-2.

FIG. 4C is a graph of the calculated band structure from both the gammaand Z points for both bulk silicon as in the prior art, and for the5/1/3/1 Si/O superlattice as shown in FIG. 3.

FIG. 5 is a schematic block diagram of a multiple-wavelengthopto-electronic device including a plurality of vertically stackedoptical devices each with a respective superlattice in accordance withthe invention.

FIG. 6 is a graph of absorption vs. energy curves for pure silicon and aplurality of superlattice structures for use in accordance with theinvention.

FIG. 7 is a schematic block diagram of an alternative embodiment of themultiple-wavelength opto-electronic device of FIG. 5 including aplurality of laterally adjacent optical devices.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout, and prime notation is used toindicate similar elements in different embodiments.

The present invention relates to controlling the properties ofsemiconductor materials at the atomic or molecular level. Further, theinvention relates to the identification, creation, and use of improvedmaterials for use in semiconductor devices.

Applicants theorize, without wishing to be bound thereto, that certainsuperlattices as described herein reduce the effective mass of chargecarriers and that this thereby leads to higher charge carrier mobility.Effective mass is described with various definitions in the literature.As a measure of the improvement in effective mass Applicants use a“conductivity reciprocal effective mass tensor”, M_(e) ⁻¹ and M_(h) ⁻¹for electrons and holes respectively, defined as:

${M_{e,{i\mspace{11mu} j}}^{- 1}\left( {E_{F},T} \right)} = \frac{\sum\limits_{E > E_{F}}{\int_{B.Z.}^{\;}{\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{i}\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{j}\frac{\partial{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}}{\partial E}\ {\mathbb{d}^{3}k}}}}{\sum\limits_{E > E_{F}}{\int_{B.Z.}^{\;}{{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}{\mathbb{d}^{3}k}}}}$for electrons and:

${M_{k,{i\mspace{11mu} j}}^{- 1}\left( {E_{F},T} \right)} = \frac{- {\sum\limits_{E > E_{F}}{\int_{B.Z.}^{\;}{\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{i}\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{j}\frac{\partial{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}}{\partial E}\ {\mathbb{d}^{3}k}}}}}{\sum\limits_{E < E_{F}}{\int_{B.Z.}^{\;}{\left( {1 - {f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}} \right){\mathbb{d}^{3}k}}}}$for holes, where f is the Fermi-Dirac distribution, E_(F) is the Fermienergy, T is the temperature, E(k,n) is the energy of an electron in thestate corresponding to wave vector k and the n^(th) energy band, theindices i and j refer to Cartesian coordinates x, y and z, the integralsare taken over the Brillouin zone (B.Z.), and the summations are takenover bands with energies above and below the Fermi energy for electronsand holes respectively.

Applicants' definition of the conductivity reciprocal effective masstensor is such that a tensorial component of the conductivity of thematerial is greater for greater values of the corresponding component ofthe conductivity reciprocal effective mass tensor. Again Applicantstheorize without wishing to be bound thereto that the superlatticesdescribed herein set the values of the conductivity reciprocal effectivemass tensor so as to enhance the conductive properties of the material,such as typically for a preferred direction of charge carrier transport.The inverse of the appropriate tensor element is referred to as theconductivity effective mass. In other words, to characterizesemiconductor material structures, the conductivity effective mass forelectrons/holes as described above and calculated in the direction ofintended carrier transport is used to distinguish improved materials.

Applicants have identified improved materials or structures for use insemiconductor devices. More specifically, the Applicants have identifiedmaterials or structures having energy band structures for which theappropriate conductivity effective masses for electrons and/or holes aresubstantially less than the corresponding values for silicon. Inaddition to the enhanced mobility characteristics of these structures,they may also be formed or used in such a manner that they providepiezoelectric, pyroelectric, and/or ferroelectric properties that areadvantageous for use in a variety of different types of devices, as willbe discussed further below.

Referring now to FIGS. 1 and 2, the materials or structures are in theform of a superlattice 25 whose structure is controlled at the atomic ormolecular level and may be formed using known techniques of atomic ormolecular layer deposition. The superlattice 25 includes a plurality oflayer groups 45 a-45 n arranged in stacked relation, as perhaps bestunderstood with specific reference to the schematic cross-sectional viewof FIG. 1.

Each group of layers 45 a-45 n of the superlattice 25 illustrativelyincludes a plurality of stacked base semiconductor monolayers 46defining a respective base semiconductor portion 46 a-46 n and an energyband-modifying layer 50 thereon. The energy band-modifying layers 50 areindicated by stippling in FIG. 1 for clarity of illustration.

The energy band-modifying layer 50 illustratively includes onenon-semiconductor monolayer constrained within a crystal lattice ofadjacent base semiconductor portions. By “constrained within a crystallattice of adjacent base semiconductor portions” it is meant that atleast some semiconductor atoms from opposing base semiconductor portions46 a-46 n are chemically bound together through the non-semiconductormonolayer 50 therebetween, as seen in FIG. 2. Generally speaking, thisconfiguration is made possible by controlling the amount ofnon-semiconductor material that is deposited on semiconductor portions46 a-46 n through atomic layer deposition techniques so that not all(i.e., less than full or 100% coverage) of the available semiconductorbonding sites are populated with bonds to non-semiconductor atoms, aswill be discussed further below. Thus, as further monolayers 46 ofsemiconductor material are deposited on or over a non-semiconductormonolayer 50, the newly deposited semiconductor atoms will populate theremaining vacant bonding sites of the semiconductor atoms below thenon-semiconductor monolayer.

In other embodiments, more than one such non-semiconductor monolayer maybe possible. It should be noted that reference herein to anon-semiconductor or semiconductor monolayer means that the materialused for the monolayer would be a non-semiconductor or semiconductor ifformed in bulk. That is, a single monolayer of a material, such assilicon, may not necessarily exhibit the same properties that it wouldif formed in bulk or in a relatively thick layer, as will be appreciatedby those skilled in the art.

Applicants theorize without wishing to be bound thereto that energyband-modifying layers 50 and adjacent base semiconductor portions 46a-46 n cause the superlattice 25 to have a lower appropriateconductivity effective mass for the charge carriers in the parallellayer direction than would otherwise be present. Considered another waysthis parallel direction is orthogonal to the stacking direction. Theband-modifying layers 50 may also cause the superlattice 25 to have acommon energy band structure. The band modifying layers 50 may alsocause the superlattice 25 to have a common energy band structure, whilealso advantageously functioning as an insulator between layers orregions vertically above and below the superlattice.

Moreover, this superlattice structure may also advantageously act as abarrier to dopant and/or material diffusion between layers verticallyabove and below the superlattice 25. These properties may thusadvantageously allow the superlattice 25 to provide an interface forhigh-K dielectrics which not only reduces diffusion of the high-Kmaterial into the channel region, but which may also advantageouslyreduce unwanted scattering effects and improve device mobility, as willbe appreciated by those skilled in the art.

It is also theorized that semiconductor devices including thesuperlattice 25 may enjoy a higher charge carrier mobility based uponthe lower conductivity effective mass than would otherwise be present.In some embodiments, and as a result of the band engineering achieved bythe present invention, the superlattice 25 may further have asubstantially direct energy bandgap that may be particularlyadvantageous for opto-electronic devices, as will be discussed furtherbelow, for example.

The superlattice 25 also illustratively includes a cap layer 52 on anupper layer group 45 n. The cap layer 52 may comprise a plurality ofbase semiconductor monolayers 46. The cap layer 52 may have between 2 to100 monolayers of the base semiconductor, and, more preferably between10 to 50 monolayers.

Each base semiconductor portion 46 a-46 n may comprise a basesemiconductor selected from the group consisting of Group IVsemiconductors, Group III-V semiconductors, and Group II-VIsemiconductors. Of course, the term Group IV semiconductors alsoincludes Group IV-IV semiconductors, as will be appreciated by thoseskilled in the art. More particularly, the base semiconductor maycomprise at least one of silicon and germanium, for example.

Each energy band-modifying layer 50 may comprise a non-semiconductorselected from the group consisting of oxygen, nitrogen, fluorine, andcarbon-oxygen, for example. The non-semiconductor is also desirablythermally stable through deposition of a next layer to therebyfacilitate manufacturing. In other embodiments, the non-semiconductormay be another inorganic or organic element or compound that iscompatible with the given semiconductor processing as will beappreciated by those skilled in the art.

It should be noted that the term monolayer is meant to include a singleatomic layer and also a single molecular layer. It is also noted thatthe energy band-modifying layer 50 provided by a single monolayer isalso meant to include a monolayer wherein not all of the possible sitesare occupied (i.e., there is less than full or 100% coverage). Forexample, with particular reference to the atomic diagram of FIG. 2, a4/1 repeating structure is illustrated for silicon as the basesemiconductor material, and oxygen as the energy band-modifyingmaterial. Only half of the possible sites for oxygen are occupied in theillustrated example.

In other embodiments and/or with different materials this one halfoccupation would not necessarily be the case, as will be appreciated bythose skilled in the art. Indeed, it can be seen even in this schematicdiagram that individual atoms of oxygen in a given monolayer are notprecisely aligned along a flat plane, as will also be appreciated bythose of skill in the art of atomic deposition. By way of example, apreferred occupation range is from about one-eighth to one-half of thepossible oxygen sites being full, although other numbers may be used incertain embodiments.

Silicon and oxygen are currently widely used in conventionalsemiconductor processing, and, hence, manufacturers will be readily ableto use these materials as described herein. Atomic or monolayerdeposition is also now widely used. Accordingly, semiconductor devicesincorporating the superlattice 25 in accordance with the invention maybe readily adopted and implemented, as will be appreciated by thoseskilled in the art.

The 4/1 repeating structure shown in FIGS. 1 and 2 for Si/O has beenmodeled to indicate an enhanced mobility for electrons and holes in theX direction. For example, the calculated conductivity effective mass forelectrons (isotropic for bulk silicon) is 0.26 and for the 4/1 SiOsuperlattice in the X direction it is 0.12 resulting in a ratio of 0.46.Similarly, the calculation for holes yields values of 0.36 for bulksilicon and 0.16 for the 4/1 Si/O superlattice resulting in a ratio of0.44.

While such a directionally preferential feature may be desired incertain semiconductor devices, other devices may benefit from a moreuniform increase in mobility in any direction parallel to the groups oflayers. It may also be beneficial to have an increased mobility for bothelectrons or holes, or just one of these types of charge carriers aswill be appreciated by those skilled in the art.

The lower conductivity effective mass for the 4/1 Si/O embodiment of thesuperlattice 25 may be less than two-thirds the conductivity effectivemass than would otherwise occur, and this applies for both electrons andholes. Of course, the superlattice 25 may further comprise at least onetype of conductivity dopant therein, as will also be appreciated bythose skilled in the art.

Indeed, referring now additionally to FIG. 3, another embodiment of asuperlattice 25′ in accordance with the invention having differentproperties is now described. In this embodiment, a repeating pattern of3/1/5/1 is illustrated. More particularly, the lowest base semiconductorportion 46 a′ has three monolayers, and the second lowest basesemiconductor portion 46 b′ has five monolayers. This pattern repeatsthroughout the superlattice 25′. The energy bandmodifying layers 50′ mayeach include a single monolayer. For such a superlattice 25′ includingSi/O, the enhancement of charge carrier mobility is independent oforientation in the plane of the layers. Those other elements of FIG. 3not specifically mentioned are similar to those discussed above withreference to FIG. 1 and need no further discussion herein.

In some device embodiments, all of the base semiconductor portions of asuperlattice may be a same number of monolayers thick. In otherembodiments, at least some of the base semiconductor portions may be adifferent number of monolayers thick. In still other embodiments, all ofthe base semiconductor portions may be a different number of monolayersthick.

In FIGS. 4A-4C, band structures calculated using Density FunctionalTheory (DFT) are presented. It is well known in the art that OFTunderestimates the absolute value of the bandgap. Hence all bands abovethe gap may be shifted by an appropriate “scissors correction.” Howeverthe shape of the band is known to be much more reliable. The verticalenergy axes should be interpreted in this light.

FIG. 4A shows the calculated band structure from the gamma point (G) forboth bulk silicon (represented by continuous lines) and for the 4/1 Si/Osuperlattice 25 shown in FIG. 1 (represented by dotted lines). Thedirections refer to the unit cell of the 4/1 Si/O structure and not tothe conventional unit cell of Si, although the (001) direction in thefigure does correspond to the (001) direction of the conventional unitcell of Si, and, hence, shows the expected location of the Si conductionband minimum. The (100) and (010) directions in the figure correspond tothe (110) and (−110) directions of the conventional Si unit cell. Thoseskilled in the art will appreciate that the bands of Si on the figureare folded to represent them on the appropriate reciprocal latticedirections for the 4/1 Si/O structure.

It can be seen that the conduction band minimum for the 4/1 Si/Ostructure is located at the gamma point in contrast to bulk silicon(Si), whereas the valence band minimum occurs at the edge of theBrillouin zone in the (001) direction which we refer to as the Z point.One may also note the greater curvature of the conduction band minimumfor the 4/1 Si/O structure compared to the curvature of the conductionband minimum for Si owing to the band splitting due to the perturbationintroduced by the additional oxygen layer.

FIG. 4B shows the calculated band structure from the Z point for bothbulk silicon (continuous lines) and for the 4/1 Si/O superlattice 25(dotted lines). This figure illustrates the enhanced curvature of thevalence band in the (100) direction.

FIG. 4C shows the calculated band structure from both the gamma and Zpoint for both bulk silicon (continuous lines) and for the 5/1/3/1 Si/Ostructure of the superlattice 25′ of FIG. 3 (dotted lines). Due to thesymmetry of the 5/1/3/1 Si/O structure, the calculated band structuresin the (100) and (010) directions are equivalent. Thus the conductivityeffective mass and mobility are expected to be isotropic in the planeparallel to the layers, i.e. perpendicular to the (001) stackingdirection. Note that in the 5/1/3/1 Si/O example the conduction bandminimum and the valence band maximum are both at or close to the Zpoint.

Although increased curvature is an indication of reduced effective mass,the appropriate comparison and discrimination may be made via theconductivity reciprocal effective mass tensor calculation. This leadsApplicants to further theorize that the 5/1/3/1 superlattice 25′ shouldbe substantially direct bandgap. As will be understood by those skilledin the art, the appropriate matrix element for optical transition isanother indicator of the distinction between direct and indirect bandgapbehavior.

Using the above-described measures, one can select materials havingimproved band structures for specific purposes. One such example wouldbe a superlattice layer 25 used in a multiple-wavelength opto-electronicdevice, such as a photovoltaic (PV) cell 20 shown in FIG. 5. The PV cell20 illustratively includes a substrate 21 and a plurality of activeoptical devices 22 a-22 n carried by the substrate and operating atdifferent respective wavelengths. In the illustrated embodiment, eachactive optical device 22 includes a first semiconductor layer 23, asuperlattice layer 25 vertically stacked on the first semiconductorlayer, and a second semiconductor layer 24 vertically stacked on thesuperlattice layer 25. In addition, in the present embodiment the activeoptical devices 25 a-25 n are vertically stacked on top of one anotheras shown.

A top or upper terminal contact 26 is on the uppermost active opticaldevice 22 a, and a bottom or backside contact 27 is on the substrate 21opposite the bottommost active optical device 22 n, as shown. In theillustrated PV cell embodiment, the upper terminal contact 26 layer ispreferably a transparent contact, such as a transparent conductive oxide(TCO) (e.g., fluorine-doped tin oxide, doped zinc oxide, and indium tinoxide), although other suitable materials may also be used. The backsidecontact 27 may be any suitable material such as metal, for example,although other contact materials may also be used.

The first and second semiconductor layers 23, 24 are oppositely doped todefine a PiN structure with the superlattice layer 25 therebetween. Inthe illustrated example, the first semiconductor layers 23 a-23 n areP-type, and the second semiconductor layers 24 a-24 n are N-type,although other configurations are also possible (e.g., these layerscould be doped with the opposite conductivity type). As such, in thepresent example the upper terminal contact 26 is a −Ve contact, and thebackside contact 27 is a +Ve contact.

As will be appreciated by those skilled in the art, the direct bandgapnature of the superlattice layer 25 makes it a more efficient opticaldetector (or transmitter) layer than silicon alone. Moreover, becausethe superlattice 25 also functions as a dopant diffusion blocking layeras discussed above, the superlattice is also well suited for use betweenP and N type layers, since the superlattice will advantageously reducedopant diffusion or creep therebetween. Further details regarding theuse of the above-described superlattice structures as dopant blockinglayers may be found in co-pending U.S. application Ser. No. 11/380,992,which is assigned to the present Assignee and is hereby incorporatedherein in its entirety by reference.

In some embodiments, it may not be necessary to include the first and/orsecond semiconductor layers 23, 24 in the active optical device 22. Thatis, portions of the superlattice 25 may instead be doped with a P/Nregion to define a PN junction with an N/P region above or below thesuperlattice 25, or both N and P regions may be doped in thesuperlattice layer. Further details on superlattice structures withregions of doping to provide PN junctions therein or in conjunction withan adjacent semiconductor layer may be found in U.S. Pat. No. 7,045,813and co-pending U.S. application Ser. No. 11/097,612, both of which areassigned to the present Assignee and are hereby incorporated herein intheir entireties by reference.

The substrate 21 may be a semiconductor substrate, such as silicon, forexample, although other semiconductors may also be used (germanium,silicon-germanium, etc.). While the substrate 21 may be monocrystallinesilicon and the layers 23-25 may be formed directly thereon in someembodiments, to provide cost savings an amorphous silicon substrate mayalso be used by epitaxially forming the layer 23 n (i.e., amonocrystalline layer) on a separate monocrystalline substrate andtransferring this layer to the amorphous substrate. One exemplaryapproach for cleaving a semiconductor and/or superlattice layer from onesubstrate and transferring/bonding the layer to another substrate is setforth in co-pending U.S. application Ser. Nos. 11/381,835 and11/381,850, both of which are assigned to the present Assignee and arehereby incorporated herein in their entireties by reference.

The superlattice layer 25 n, which also advantageously has a crystallinestructure as discussed above, and the semiconductor layer 24 n may thenbe formed on the crystalline layer 23 n by epitaxial deposition,following by the deposition of the various layers 23-25 of the remainingoptical detectors 22, as will be appreciated by those skilled in theart. An alternative approach is to form one or more of the devices 22a-22 n on a separate crystalline substrate and then transfer thedevice(s) to the substrate 21, as will also be appreciated by thoseskilled in the art. In other embodiments, non-semiconductor substratesmay also be used. For example, to provide further cost savings a glasssubstrate may be used, for example, through the above-describedtransferring process. However, other suitable substrate materials knownto those skilled in the art may also be used.

As noted above, each of the active optical devices 22 a-22 nadvantageously operates at different respective wavelengths λa-λn.Stated alternatively, each of the active optical devices 22 a-22 n(i.e., optical detectors in the present PV cell embodiment) are “tuned”to advantageously absorb or transmit, in the case of opticaltransmitters, light of a different respective wavelength. Thus, in thepresent example, the PV cell advantageously provides enhanced efficiencywith respect to a typical single layer solar cell. This is because thereare multiple detectors 22 a-22 n that generate electricity based upondifferent wavelengths of sunlight incident on the device 20, yet over asame amount of surface area as the single layer device, as will beappreciated by those skilled in the art. Accordingly, a greater quantityof energy production is achieved per the same amount of surface area.

One approach for tuning the superlattices 25 a-25 n to operate atdifferent wavelengths is to form the superlattices with different layerconfigurations. More particularly, theoretical calculations usingGeneralized Gradient Approximation (GGA) and Screened Hartree-Fock (SHF)methods both indicate tunable wavelengths as a function of layerspacing/configuration. Similar calculations using just GGA also indicatethat wavelength tuning is possible by varying the coverage of thenon-semiconductor monolayers in the respective superlattices, asdiscussed above.

By way of example, using SHF methodology the calculated differences inbandgap between an 8/1 Si—O superlattice structure and a 3/1/5/1 Si—Osuperlattice structure relative to silicon alone are 0.21 eV and 0.58eV, respectively. These shifts in bandgap lead to respective wavelengthoffsets of 217 nm and 469 nm lower than silicon. Furthermore, becausethe superlattice structures 25 are substantially direct bandgapmaterials, this leads to the ability to not only detect/generate light,but to do so at different wavelengths when each superlattice layer has arespective different bandgap than the other superlattice layers, as inthe above example.

The optical detectors 22 a-22 n are configured to provide an outputequal to a sum of photocurrents therefrom. In operation, each photon ofsunlight creates an electron-hole pair that, if formed in the PNjunction region and experiences an electric field, separates and movesin opposite directions resulting in current flow. Voltage can either bemeasured across the detectors 22 a-22 n (for an open-circuit) or acurrent (with a closed-circuit), but practically speaking, the poweroutput (which is the product of the voltage and current) is preferablyoptimized for the given application, as will be appreciated by thoseskilled in the art.

The higher bandgap superlattice layer 25 generates a larger voltageoutput for the photovoltaic (PV) cell, and hence, delivers more powerfor an equivalent absorption efficiency (i.e. current). While there maypotentially be some trade-off between closed-circuit current andopen-circuit voltage when creating the PV cell 20, the stacking ofseveral active optical detectors 22 provides a multilayer PV cell thatcaptures an increased amount of electromagnetic (EM) radiation across awide spectrum of energies.

While certain prior art PV cell approaches have attempted to use otherstacked detector configurations to capture light of differentwavelengths, particularly for solar cells made with III-V alloys, onesignificant drawback of such arrangements is that the strain frommismatched lattices damages the crystals. However, applicants theorizewithout wishing to be bound thereto that this shortcoming of such priorart structures may be alleviated by using superlattice layers 25 asdescribed above with different superlattice spacings and differentdoping levels, as will be appreciated by those skilled in the art.

Turning now additionally to FIG. 6, absorption versus energy curves forvarious superlattice structures and silicon are now compared. The “areaunder the curve” shows that PV cells made with the above-describedsuperlattice layers will absorb more energy per layer thickness in thelower energy region (i.e., 1-2 eV) than pure silicon. In FIG. 6, thesuperlattice SL1 is a 4-1 Si/O structure, the superlattice SL2 is a 8-1Si/O structure, and the superlattice SL3 is a 12-1 Si/O with 50%coverage structure. Thus, even a single cell design using a superlattice25 layer can be made thinner than with silicon, resulting in weightsavings for critical applications. One example where weight may be ofparticular importance is in space-based solar cells, e.g., onsatellites, etc. In addition, the superlattice 25 has increasedabsorption in the lower energy region, which is also particularlyadvantageous in a space-based applications, since in outer space asubstantial amount of solar energy is located in the 0.5 eV to 2 eVspectral range.

Referring additionally to FIG. 7, another embodiment of amultiple-wavelength opto-electronic device 20′ illustratively includeslaterally adjacent active optical devices 22 a′ and 22 b′, although morethan just the two illustrated devices may be used. The active opticaldevices 22 a′ and 22 b′ may be optical transmitters, optical detectors,or a combination of both optical detectors and transmitters, as will beappreciated by those skilled in the art. The active optical devices 22a′ and 22 b′ may be used in lasers, optical communications devices, etc.The optical devices 22 a′ and 22 b′ may be coupled to respectivewaveguides, or to a common waveguide using wave division multiplexing(WDM) techniques known to those skilled in the art, for example. Furtherdetails on implementing the above-described superlattice films inopto-electronic applications are provided in U.S. application Ser. Nos.10/936,903, 10/936,933, 10/936,913, and 10/937,072, which are assignedto the present Assignee and are hereby incorporated herein in theirentireties by reference.

In the present example, the active optical devices are also implementedon a semiconductor (e.g., silicon) on insulator (SOI) substrate 21′having an insulating (e.g., oxide) layer 28′ thereon. Further details onusing the superlattice 25 in SOI implementations are provided in theabove-noted co-pending U.S. application Ser. Nos. 11/381,835 and11/381,850, and also in co-pending U.S. application Ser. Nos. 11/428,015and 11/428,003, which are also assigned to the present Assignee and arehereby incorporated herein in their entireties by reference. Moreover,an insulating or shallow trench isolation (STI) region 29′ isillustratively included between the active optical devices 22 a′, 22 b′.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of theinvention.

1. A multiple-wavelength opto-electronic device comprising: a substrate;and a plurality of active optical detectors carried by said substrateand tuned to different respective operating wavelengths; each activeoptical detector comprising a superlattice comprising a plurality ofstacked groups of layers, and each group of layers comprising aplurality of stacked semiconductor monolayers defining a basesemiconductor portion and at least one non-semiconductor monolayerthereon.
 2. The multiple-wavelength opto-electronic device of claim 1wherein said at least one non-semiconductor monolayer is constrainedwithin a crystal lattice of adjacent base semiconductor portions, and atleast some semiconductor atoms from opposing base semiconductor portionsare chemically bound together through the at least one non-semiconductormonolayer therebetween.
 3. The multiple-wavelength opto-electronicdevice of claim 1 wherein each active optical detectors furthercomprises first and second semiconductor regions on opposing sides ofsaid superlattice and having opposite conductivity types.
 4. Themultiple-wavelength opto-electronic device of claim 1 wherein saidactive optical detectors are stacked in a vertical direction.
 5. Themultiple-wavelength opto-electronic device of claim 1 wherein saidactive optical detectors are laterally adjacent one another.
 6. Themultiple-wavelength opto-electronic device of claim 1 wherein saidplurality of superlattices have different numbers of semiconductormonolayers in their respective base semiconductor portions.
 7. Themultiple-wavelength opto-electronic device of claim 1 wherein saidactive optical detectors are configured to provide an output equal to asum of photocurrents therefrom.
 8. The multiple-wavelengthopto-electronic device of claim 1 wherein each superlattice has asubstantially direct bandgap.
 9. The multiple-wavelength opto-electronicdevice of claim 1 wherein each superlattice has a different respectivebandgap.
 10. The multiple-wavelength opto-electronic device of claim 1further comprising at least one contact coupled to said plurality ofactive optical detectors.
 11. The multiple-wavelength opto-electronicdevice of claim 1 wherein said substrate comprises a semiconductor. 12.The multiple-wavelength opto-electronic device of claim 1 wherein saidsubstrate comprises a non-semiconductor.
 13. The multiple-wavelengthopto-electronic device of claim 1 wherein said substrate comprisesglass.
 14. The multiple-wavelength opto-electronic device of claim 1wherein each base semiconductor portion comprises silicon.
 15. Themultiple-wavelength opto-electronic device of claim 1 wherein each basesemiconductor portion comprises a base semiconductor selected from thegroup consisting of Group IV semiconductors, Group III-V semiconductors,and Group II-VI semiconductors.
 16. The multiple-wavelengthopto-electronic device of claim 1 wherein each non-semiconductormonolayer comprises oxygen.
 17. The multiple-wavelength opto-electronicdevice of claim 1 wherein each non-semiconductor monolayer comprises anon-semiconductor selected from the group consisting of oxygen,nitrogen, fluorine, and carbon-oxygen.
 18. A multiple-wavelengthopto-electronic device comprising: a substrate; and a plurality ofactive optical detectors carried by said substrate and stacked in avertical direction, said active optical detectors tuned to differentrespective operating wavelengths; each active optical detectorcomprising a superlattice comprising a plurality of stacked groups oflayers, and each group of layers comprising a plurality of stackedsemiconductor monolayers defining a base semiconductor portion and atleast one non-semiconductor monolayer thereon wherein said at least onenon-semiconductor monolayer is constrained within a crystal lattice ofadjacent base semiconductor portions, and at least some semiconductoratoms from opposing base semiconductor portions are chemically boundtogether through the at least one non-semiconductor monolayertherebetween.
 19. The multiple-wavelength opto-electronic device ofclaim 18 wherein each active optical detector further comprises firstand second semiconductor regions on opposing sides of said superlatticeand having opposite conductivity types.
 20. The multiple-wavelengthopto-electronic device of claim 18 wherein said plurality ofsuperlattices have different numbers of semiconductor monolayers intheir respective base semiconductor portions.
 21. Themultiple-wavelength opto-electronic device of claim 18 wherein saidactive optical detectors are configured to provide an output equal to asum of photocurrents therefrom.
 22. The multiple-wavelengthopto-electronic device of claim 18 wherein said substrate comprises asemiconductor.
 23. The multiple-wavelength opto-electronic device ofclaim 18 wherein said substrate comprises a non-semiconductor.
 24. Themultiple-wavelength opto-electronic device of claim 18 wherein each basesemiconductor portion comprises silicon.
 25. The multiple-wavelengthopto-electronic device of claim 18 wherein each non-semiconductormonolayer comprises oxygen.